It is known to provide an elastic mount between an internal-combustion engine and a vehicle chassis to damp both high and low frequency vibrations and prevent direct transmission of vibrations of the engine to the chassis.
In systems in which an internal-combustion engine is to be elastically mounted on a vehicle chassis, vibrations of low frequency should be damped to a lesser extent to prevent work hardening of the elastic material and improve acoustic insulation in the audible range.
To this end it is known to provide an elastic mount which uses in part the deformation of an elastomeric body or material and in part hydraulic damping in which a movable member is provided with an orifice separating a pair of chambers and in which hydraulic damping is effected by forcing fluid from one chamber to the other through the orifice.
An elastic mount having an enclosed gas chamber or cushion is disclosed in the German utility model (Gebrauchsmuster) DT-GM No. 18 05 392. In this system, the compression of the mount reduces the volume of the chamber delineated in part by the elastomeric material and forces the damping fluid through a throttle orifice in a separating membrane within the chamber. The chamber is thus defined in part by a rigid wall which forms one of the compartments into which the fluid is forced by reduction in the volume of the gas cushion. Upon expansion, a pressure drop in the gas cushion enables the damping fluid to flow from the compartment delivered by the rigid wall into the compartment by the elastomeric wall.
A mount of this type, which must include a gas cushion, is expensive to fabricate. Because of permeation of the flexible wall by the gas or diffusion of the gas through the latter, the damping effect after some time develops indeterminate values.
Since a mount having elastomeric members and subjected to high stress can become very hot and the volume and pressure of the enclosed gas is dependent upon the temperature, there is a disadvantageous effect upon the stiffening of the mount and its damping effect between cold and warm states.
With rapid compression of the mount, corresponding to high vibration frequencies, the liquid exchange between the two chambers can no longer occur completely during the compression stage and thus the damping effect is reduced. The stresses also result in hardening or stiffening of the elastomeric members at a rapid rate.
An elastic mount having two working compartments separated by a membrane and defined at least in part by elastomeric walls is disclosed in German utility model (Gebrauchsmuster) DT-GM No. 76 32 663. In this system, the separating membrane is formed with a throttle orifice and is yieldable in the axial direction in the region of this orifice to reduce the damping effect with higher frequency vibrations.
In this case, the damping liquid has less of a supporting function so that the elastomeric walls of the mount must be made sufficiently stiff to take up the supporting function. This can be achieved by making these walls relatively thick. A mount of this type uses considerable elastomeric material.
The elastomeric elements which delineate the outer walls of the compartments change shape during compaction of the mount and generate a pressure difference between the two compartments. This pressure difference is reduced by yielding of the membrane and thus the damping which can be effected by passage of the damping liquid through the orifice, is reduced. This reduction in the damping occurs even with small pressure differentials characterizing low frequency vibrations where a high degree of damping is desired. Naturally, this is a disadvantage.